1. Field of the Invention
The present invention relates to a silicon nitride sintered body; a process for producing the same; a ceramic heater employing the silicon nitride sintered body serving as a substrate; and a glow plug containing the ceramic heater. More particularly, the present invention relates to a silicon nitride sintered body which can suppress reduction of a sintering aid such as silicon oxide occurring in a reducing atmosphere formed by a carbonaceous component during firing, and which sintered body has consistent qualities such as excellent bending strength without variation and is sufficiently densified through to the surface, and to a process for producing the sintered body. The present invention also relates to a ceramic heater employing the silicon nitride sintered body, which heater can be utilized for a variety of uses and whose properties such as mechanical strength and electrical resistance demonstrate negligible variation, and to a glow plug containing the ceramic heater serving as a heat source and employable in diesel engines.
2. Description of Related Art
Silicon nitride is sintered by the action of a liquid phase formed from a sintering aid during heating. The melting point of the formed liquid phase is known to vary in relation to the compositional ratio between a main sintering aid such as MgO, Al2O3xe2x80x94Y2O3, or RE2O3 (RE: rare earth element) and SiO2, which serves as an additional sintering aid and is contained in silicon nitride raw material. The sinterability is greatly affected by variations in the melting point, thereby varying the quality of the produced sintered body.
When a silicon nitride sintered body is produced through hot pressing, a pressing die made of graphite is typically employed, thereby providing a reducing atmosphere for firing. Therefore, an oxide sintering aid, particularly silicon oxide partially undergoes reduction, and the compositional proportions of sintering aids vary. As a result, sinterability is greatly affected as described above, and sintered bodies having consistent physical properties are difficult to produce, because of intra-lot and inter-lot variations in quality of the produced sintered bodies. In addition, since a portion of the raw material in the vicinity of the pressing die is insufficiently sintered, a silicon nitride sintered body fully densified through to the surface may not be produced.
The present invention has been made to solve the aforementioned problems. The silicon nitride sintered body of the first aspect of the invention contains a carbide which is converted from a sintering aid protecting agent (hereinafter referred to as a xe2x80x9cprotecting agentxe2x80x9d) which prevents reduction of a sintering aid due to a carbonaceous component contained in a reducing atmosphere during hot press firing.
As described in the second aspect of the invention, examples of protecting agents having a high melting point include at least one species from among Ta, W, Mo and compounds thereof. Of these, elements, nitrides, and silicides, such as W, TaN and MoSi2, which are readily converted to carbides thereof in a reducing atmosphere, are preferable.
As described in the third aspect of the invention, the sintering aid contains silicon oxide and the protecting agent contains a metal and/or metallic compound which can be converted to carbide more easily than silicon oxide.
As described in the fourth aspect of the invention, sintering aids including a rare earth element oxide and silicon oxide are employed and reduction of the sintering aids is sufficiently prevented, thereby efficiently producing a silicon nitride sintered body having a grain boundary phase containing RE2SiO5 and/or RE2Si2O7.
In the fifth aspect of the invention, there is provided a process for producing a silicon nitride sintered body, which process comprises obtaining a compact from a material containing silicon nitride raw material powder, sintering aid powder and a protecting agent; placing the compact in a pressing die; placing the pressing die holding the compact in a firing furnace and hot press firing.
In the sixth aspect of the invention, there is provided a ceramic heater containing a substrate and a resistance heater, wherein the substrate is formed of a silicon nitride sintered body as recited in any one of the first to fourth aspects of the invention.
The seventh aspect of the invention comprehends a glow plug containing a ceramic heater as recited in the sixth aspect of the invention.
Accordingly, it is an object of the present invention to provide a silicon nitride sintered body which can suppress reduction of a sintering aid occurring when a firing atmosphere becomes a reducing atmosphere due to a carbonaceous component, particularly when a pressing die formed of graphite is used, and which silicon nitride sintered body has consistent qualities such as excellent bending strength without variation and is sufficiently densified through to the surface.
Another object of the present invention is to provide a process for producing the sintered body.
Still another object of the present invention is to provide a ceramic heater which employs the silicon nitride sintered body as a substrate and which has reduced variation in properties such as mechanical strength and electrical resistance.
Yet another object of the present invention is to provide a glow plug containing the ceramic heater as a heat source.